The invention relates generally to systems and methods for converting gases to liquids. More particularly, the invention relates to converting natural gases to liquids using catalytic partial oxidation.
Natural gas or other gaseous hydrocarbons are normally converted into a liquid form, such as longer-chain hydrocarbons, using a large scale gas-to-liquid process. Methane-rich gases are converted into liquid fuels using syngas as an intermediate, as in a Fischer Tropsch (F-T) process.
An F-T process is a set of chemical reactions that convert a mixture of hydrogen (H2) and carbon monoxide (CO) into liquid hydrocarbons. The mixture of H2 and CO may be obtained by subjecting natural gas to partial oxidation. Catalytic partial oxidation (CPO) is one of such partial oxidation processes that oxidize the natural gas in the presence of oxygen, over a catalyst. Treatment of natural gas in a CPO process normally yields H2, CO, carbon dioxide (CO2), and water. The H2 and CO can be used in the subsequent F-T process.
In the CPO process, a pure oxygen input is typically used to obtain a cleaner (without nitrogen dilution) output, so as to obtain higher carbon conversion efficiency from CO to hydrocarbons in an F-T reaction. However, producing oxygen by separating oxygen from air typically requires an air separation unit (ASU), which further requires an input of energy. The additional energy requirement for producing oxygen for the CPO process, and the significant capital investment for producing a mixture of H2 and CO (a mixture commonly referred to as syngas) for the F-T process increases the cost of producing liquids from the gases.
As noted above, the syngas is chemically reacted in the F-T reaction over a catalyst to produce liquid hydrocarbons and other byproducts. However, the H2-to-CO ratio obtained from a typical CPO process may not be the optimal ratio for carrying out the F-T reaction. Normally, the H2-to-CO ratio obtained from a partial oxidation reaction may be lower than what is required for the F-T reaction. The ratio of H2-to-CO may be adjusted before entering the F-T system, by using a water gas shift reaction or alternatively, carrying out steam methane reforming (SMR), instead of CPO. The water gas shift reaction involves reaction of water with CO to produce H2 and CO2, hence increasing the H2-to-CO ratio. The excess carbon dioxide may be removed before the gases enter the F-T system.
The SMR reaction is an alternative method to produce syngas with a higher H2-to-CO ratio (syngas ratio). In this process, methane is reacted with water to produce H2 and CO, with a syngas ratio of about 3.0. This ratio is higher than is required by the F-T reaction. Further, the SMR reaction is an endothermic reaction. Therefore, a portion of the natural gas is usually combusted, to provide energy for the SMR reaction. Since a portion of the feed is combusted instead of being used to generate H2 and CO, the overall conversion efficiency of the SMR reaction is undesirably reduced. In general, the requirement for external heating, and the higher syngas ratio, are two primary drawbacks in using SMR for gas-to-liquid conversion.
Formation of liquid hydrocarbons such as alkanes in the F-T process is desirable. However, methane formation from the F-T reaction is generally not desirable. The F-T process is generally operated in the temperature range of about 190° C.-350° C. Higher temperatures lead to faster reactions and higher conversion rates. However, the higher temperatures also favor methane production.
Another method of increasing F-T reaction rates and conversion is by increasing the pressure within the F-T system. A typical method for increasing the pressure within an F-T system includes compressing the syngas before entering the F-T system. However, pressurizing the syngas before entering the F-T system requires more energy input to the overall system, thereby increasing the cost of gas-to-liquid conversion.
Therefore, there is a need to reduce the energy input to the overall process of converting gases to liquids. A process that requires no energy input (or a greatly-reduced energy input) for the ASU, for syngas producing reaction, and/or for syngas compression, may decrease the overall cost of producing hydrocarbon liquids from natural gas. Furthermore, eliminating the step of balancing the H2-to-CO ratio may benefit the overall process.